The present invention relates to telephone circuits, which may be monolithically integrated, designed to form an interface between a subscriber's telephone line and exchange control components, and in particular to a telephone circuit for the supplying of ringing signals to a telephone line and the reliable detection of an off the hook condition during ringing.
A subscriber's telephone set is generally connected to a telephone exchange by a two-wire line, to whose terminals both the speech circuit and the ringer of the telephone set itself are respectively connected in parallel.
The speech circuit is connected in series with a switch which is closed only when an off the hook condition takes place, while a capacitor is connected in series with the ringer and is designed to block the direct current component from the line.
A subscriber's telephone line is supplied by a DC voltage generator in series with which there is connected an AC voltage generator which constitutes the source of ringing signals when the exchange components command the supply of these signals to the subscriber.
The exchange control components also determine the ringing rhythm at which the ringing signals are sent to the subscriber's set.
The ringing signals, which have a sinusoidal wave shape and are of a predetermined duration, are supplied to the line in succession spaced by predetermined pauses with no signal.
When an off the hook condition occurs during the supply of a ringing signal, a DC current component is superimposed on the line on the AC ringing current. The AC ringing current is absent both during the pauses between the ringing signals and during the time intervals between one call and the next by the subscriber.
In the modern telephone exchanges, an off the hook condition is detected by a circuit means of an electronic type designed to detect line current variations and to detect the presence of DC current on the line due to an actual off the hook condition, even when the AC current of a possible ringing signal is superimposed thereon.
The sinusoidal ringing signals generally have a frequency of between 16 and 66 Hz and working voltage values which are much higher (60-80 V) than those of normal telephone speech signals and are thus, if they are not immediately discontinued at the time when the phone is taken off the hook, converted by the speech circuit of the subscriber's telephone set into high intensity acoustic signals which may damage the subscriber's hearing and the telephone itself.
A telephone circuit for the detection of an off the hook condition during ringing must therefore have extremely short response times to make it possible to immediately discontinue the supply of ringing signals when an off the hook condition takes place during ringing.
In certain circumstances, however, variations in the line current due to other factors (for example, interference generated by other subscribers) could be interpreted as line current variations following the supply of a DC current component due to an off the hook condition. In order to ensure in all cases that an off the hook condition has been reliably detected, a second check may be carried out, immediately after the discontinuation of the supply of ringing signals, to ascertain whether a DC current is present on the line and to confirm that an off the hook condition by the subscriber has taken place. This solution is disclosed in Italian Pat. Application 30050 A/76 for telephone systems of a conventional type, comprising ringing apparatus with circuits which cannot be monolithically integrated.
It should be noted, however, that the detection of an off the hook condition is not in practice absolutely reliable with this method since any discontinuation of the ringing signals during the transmission of one of these signals may cause an instantaneous voltage and line current variation with the generation of harmonics likely to prevent a reliable check immediately thereafter.